1. Field of the Invention
This invention relates to peel-apart film of the self-developing type, in general, and to an improved film format which will both facilitate the automated handling of such film during film processing and reduce film processing costs, in particular.
2. Description of the Prior Art
Self-developing, peel-apart film units are well known in the field of instant photography. Each film unit comprises a negative or photosensitive sheet for forming a negative image of a subject, an image receiving sheet for forming a positive subject image and a rupturable pod of processing liquid. A positive image is formed on the positive sheet by means of a well-known diffusion transfer process after the pod containing the processing liquid is ruptured and its contents spread between said positive and negative sheets.
Film units of the aforementioned type are typically exposed in portable, multiple-exposure, instant-type photographic cameras or processed in large format film processing equipment. When employed in a camera as many as ten film units are provided in a single lighttight film pack, in a stacked relation, with the positive sheet on one side and the negative sheet on the other side of a pressure plate located within the film pack. Pull tabs are attached to one or both of the leading ends of the positive and negative sheets. After exposing an outermost negative sheet at a film plane of the camera a first tab attached to the negative sheet is withdrawn from the camera forcing the exposed negative to be turned 180.degree. about the pressure plate and into superposition with the positive sheet. This movement causes a second tab attached to the forward ends of the positive and negative sheets to enter the bite of a pair of pressure applying spread rollers. A combination of the angle at which the first tab is attached and the force applied by a camera operator causes the first tab to detach from the negative sheet at this time.
After the first tab is so detached, the operator pulls the second tab so as to advance the positive and negative sheets between the pair of spread rollers in a superposed relation. At the beginning of the advancement of the positive and negative sheets the spread rollers apply pressure to a pod containing a processing liquid located at the leading ends thereof, thereby rupturing same, and spreading its contents between the superposed sheets in a thin uniform layer as the sheets are advanced therebetween. A positive image is formed on the positive sheet outside of the camera by a diffusion transfer process. After a required interval of time the positive image is peeled away from its negative and the negative, along with the second tab attached thereto are discarded. A more detailed description of this particular type of peel-apart film may be found at pages 185-187 of a book entitled, "Camera Technology" by Norman Goldberg published in 1992 by the Academic Press.
The above-described peel-apart film format has several disadvantages, especially if considered for use in certain photographic apparatus. One disadvantage is film cost. In addition to the multiple tabs and the relatively large amounts of leader material required for each film unit, which necessarily increases material costs, economies of scale are limited by the fact that both a positive and a negative sheet must be included within a single film cassette. Including significantly larger numbers of film units within a single cassette in order to take advantage of such economies, would substantially increase cassette size and thereby make such a cassette unwieldy for use by a camera operator. Another disadvantage is that this type of film format is not susceptible of inclusion in highly automated film processing equipment. Processing a film unit by pulling certain tabs at various times in the film processing cycle is clearly a processing technique that is inherently manual.
As noted above, self-developing, peel-apart film units of the aforementioned type are also processed in large format, film processing equipment. These film units are similar in format and construction to the large format color film marketed by Polaroid Corporation under the trade designation Type 800 Land Film. This large format film and a portion of the equipment employed to process such film are shown in prior art drawing FIGS. 1 and 2. As shown in FIGS. 1 and 2, a peel-apart, self-developing, large format film unit 10 comprises a photosensitive or negative sheet element 12 and a nonphotosensitive or positive sheet element 14. The negative sheet element 12 includes a leader 16 with a rear-facing, pocket-like opening 18 having a forward projecting tab 20. An opaque envelope 22, sealed on three sides, encloses the bottom or photosensitive surface 23 of the negative sheet element 12 to prevent its exposure to ambient light. The positive sheet element 14 has a nonphotosensitive image-receiving or coating layer 24 and a rupturable pod 25 attached to a leader 26 which is folded back on itself on the image-receiving layer side thereof and is adhesively maintained in this folded condition. A slot 28 formed in the fold of the leader 26 is adapted to receive the tab 20 of the negative sheet element 12.
A lighttight cassette 30 (FIG. 2) is provided for enclosing the opaque envelope 22 and the negative sheet element 12 included therein. The cassette comprises a base housing member 32 and a cover 34 having an exposure opening therein (not shown) which is pivotally movable with respect to the base 32 about a hinge 36. A tab 38 is mounted in a fixed position within the base member 32 of the cassette 30. To load the cassette 30 the opaque envelope 22 together with the negative sheet element 12 included therein is placed within the base housing member 32 such that the tab 38 enters the pocket-like opening 18 of said negative sheet element 12. In this position the tab 20 portion thereof projects beyond the forward end of the base housing member 32 and a portion of the envelope 22 extends beyond the rear or opposite end thereof via a lighttight opening 40. When the cover 34 is placed in its closed position, a lighttight chamber is formed within the cassette 30. The envelope 22 is then removed from the negative sheet element 12 through the lighttight opening 40 in the cassette 30. Negative sheet element 12 is prevented from being withdrawn with the opaque envelope 22, because of the pocket-like opening 18 of the negative sheet leader 16 being engaged by the fixedly mounted tab 38.
The cassette 30 includes a displacable dark slide (not shown) in the cassette cover 34. The dark slide is temporarily displaced in order to subsequently expose the photosensitive layer of the negative sheet element 12 to scene light through exposure control and lens systems in an appropriate photographic camera, to form a subject image thereon. The dark slide is then replaced over the photosensitive layer and the cassette 30 with the negative sheet element 12 included therein is removed from the camera.
To process an exposed negative, the positive sheet element 14 is placed in a generally flat, horizontal tray 42 of a conventional large format film processor 44 with its image-receiving layer 24, rupturable pod 25 and the folded portion of the leader 26 facing upward. The cassette 30 is placed on top of the processor tray 42 with the exposed photosensitive layer of the negative sheet element 12 facing the image-receiving layer of the positive sheet element 14. Cassette 30 is then moved forward by a processor operator until the forward projecting tab 20 of the negative sheet element 12 enters the slot 28 in the positive sheet element leader 26 to the point where the shoulder portions 46A, 46B thereof engage the crease in the fold formed in the leader 26 and where the leading end of said tab 20 engages the bite of a pair of motor driven processing rollers 48A, 48B (motor not shown). When the motor that drives the processing rollers is energized, the negative sheet element tab 20 together with the engaged positive and negative sheet elements 12 and 14 are drawn into the rollers 48A, 48B to initiate film processing. The initial movement of the tab 20 by the rollers 48A, 48B causes the photosensitive layer on the negative sheet element 12 and the image-receiving layer on the positive sheet element 14 to be moved into longitudinal registration with one another. As the negative and positive sheet elements 12 and 14 are drawn through the spread rollers 48A, 48B, the pod of processing liquid 25 is ruptured thereby and its contents are uniformly spread between the photosensitive layer on the negative sheet element 12 and the image-receiving layer on the positive sheet element 14 to initiate the formation of a positive image. When the required amount of image formation time has elapsed the positive sheet is peeled from the negative sheet and the negative sheet together with a mask and leaders from both sheets are discarded.
Large format film material costs are somewhat lower than that of the above-described multiple pull-tab, peel-apart film due primarily to a reduction in the size of the leader on the positive sheet element 14. However, the leader design of both such negative sheet elements is comparable which should make the cost of comparably sized negatives more or less equivalent. More importantly though, the format employed in this large format type of film has inherent undesirable features which render it unsuitable for use with highly automated film processing equipment for peel-apart self-developing film. One such undesirable feature is the location of the rupturable pod of processing liquid 25 on the positive sheet element 14. In addition to its susceptibility to being damaged by the tab 20 of the negative sheet element 12 when tab 20 is inserted into the slot 28 in the leader 26 of the positive sheet element 16, the location of the pod 25 sometimes causes the leading end of the tab 20 to become trapped between said pod 20 and an adjacent surface of the leader 26 as it is being moved toward the leader slot 28 by the equipment operator to thereby cause a film unit misfeed within the film processing equipment.
Another undesirable feature associated with the above-described large format color film is the requirement that one sheet element thereof be moved or transported by the other when both are moved into the bite of a pair of spread rollers to initiate film processing. As described above, in order to produce such simultaneous movement, each large format film sheet element requires a different type of force transmitting means. The negative sheet element is moved by the spread rollers 48A, 48B when the tab 20 of the negative sheet element 12 enters the bite thereof, whereas the positive sheet element is moved only when its leader 26 is engaged by shoulder portions 46A, 46B of the negative sheet leader 16. Requiring two different types of force transmitting means within a film processor wherein, for example, it is desirable to introduce both positive and negative sheet elements of a self-developing peel-apart film unit into the processor from separate film cassettes, could increase film processor complexity and manufacturing costs over an arrangement where only a single type of force transmitting means need be employed.
At this point it should be noted that self-developing integral film such as that marketed by Polaroid Corporation under the trade designation "Spectra Film" is also moved into a developer liquid spread system by a force transmitting member incorporated within certain photographic apparatus. The force transmitting member (sometimes referred to as a "pick") engages the trailing edge of the integral film unit, after film exposure, and moves the leading edge thereof into the bite of a pair of processing rollers. However, this method of integral film movement would not be appropriate for moving sheet elements of a peel-apart film unit, such as that described above, into a developer spread System. This is so because a negative sheet element, and to a lesser extent a positive sheet element, lack the degree of stiffness necessary for a force transmitting member to be able to consistently position one sheet element of a peel-apart film unit in an overlaying relation with respect to another sheet element, for subsequent film unit processing, with the application of a force to the trailing edge of either of said sheet elements.